The present invention is directed to the preparation of a class of fixed-charge membranes that (by employing modifications of composition and procedure) are applicable as ion-exchange membranes for electrodialysis, as bipolar membranes for water-splitting processes, as selective membranes for dialysis processes and, more particularly, as ultrafiltration membranes in pressure-driven processes.
The conventional solvent-type reverse osmosis membranes require a substantial hydrostatic pressure be applied to the solution to be purified on the side of the membranes that contains the solvent or the desired component through the membrane leaving the undesirable component or solute behind because the membrane is relatively impermeable to the undesirable solute. Reverse osmosis processes have the disadvantage in that they require very high pressures, for example, pressures of about 600 to 1000 psi above the reversible osmotic pressures are commonly employed. These high pressures present certain practical difficulties that must be overcome. Additionally, reverse osmosis membranes have comparatively limited applications because they are relatively nonselective and customarily permeable only to the solvent, e.g., water. Accordingly, reverse osmosis membranes are not ordinarily especially useful in separating the dissolved components of a solution as is often required in the treatment of mixtures. Further, when used to treat solutions containing high molecular weight material such as proteins or dissolved organic matter, they tend to get clogged or readily poisoned as the organic matter is selectively absorbed by the membrane. Organic matter can either coat the surface or make it hydrophobic and thereby prevent the transport of water across it. Specifically, as in desalination and other water renovation processes, the organic matter can penetrate the membrane, disrupt the hydrogen-bonded water structure, which is apparently responsible for the selective action of the membranes, and thereby destroy either the selective action of the membrane or its high flux rate, or both.
The conventional ion-exchange membranes employed in electrodialysis are known to possess useful electrochemical properties. However, ion-exchange membranes have the drawback of being relatively expensive because they are customarily produced by time-consuming and conventional chemical reactions. Therefore, it is the practice to use rather high current densities to keep investment costs to a minimum, and for this reason the power consumption of electrodialysis devices is very substantial. Also, with electrodialysis, as carried out at high current densities, many undesirable side effects are known to occur; for example, in sea water desalting, electrodialysis devices frequently use a high current density with the result that the power consumed is 150 kilowatt hours per 1,000 gallons desalted in contrast to the theoretical power requirement of approximately 3 kwh. Further, conventional ion-exchange membranes possess a low hydraulic permeability so they cannot be used for pressure-driven processes as those of reverse osmosis or ultrafiltration. Additionally, ion-exchange membranes usually cannot be sealed together to form bipolar or multipolar membranes, and their low hydraulic permeability precludes their being used in bipolar water-splitting processes which use electric currents high enough to be practical.
Ultrafiltration membranes capable of separating dissolved solved solutes of molecular weights ranging from those of the common salts to proteins have been made in the past by mixtures of polyelectrolytes held together by purely ionic bonds as complexes of polyacids and polybases, deposited upon a hydrophobic, porous matrix, or they are porous films of largely hydrophobic polymers usually formed by coagulation techniques. The polysalt complexes are not chemically (covalently) crosslinked, and they always contain a mixture of fixed positive and negative charges. As such, they can interact with dissolved species, which are both anionic and cationic, and become fouled thereby. The hydrophobic membranes of this class also adsorb fouling agents of hydrophobic character and similarly become fouled, and their utility is inhibited thereby.
The purpose of the membranes of the present invention is directed to a novel method for preparing membranes and to the membranes produced thereby and which are useful for a variety of applications. For example, when used as ultrafiltration membranes, they can be used to separate dissolved materials using a variant of the more familiar reverse osmosis process, except, instead of requiring very high pressures, a high flux rate can be effected by the imposition of a moderate hydrostatic pressure, e.g., of the order of 25 to 150 psi. When employed as bipolar membranes they may be used for the production of acid and base by electrodialytic water-splitting. They may also be used effectively as ion-exchange membranes of low cost. Additionally, they are useful in separations involving passive diffusion of materials across the membrane by virtue of concentration gradients in passive dialysis.
As contrasted with cast membranes of ionic character, as described in the prior art, the novel membranes of the invention are chemically crosslinked by covalent bonds to the degree heretofore not attainable, so they retain their particularly desirable properties for prolonged periods of time, such crosslinking having been achieved from a membrane cast from a solution whose homogeneity was maintained over a wide range of compositions.